Why is sleep necessary for rest and rejuvenation?

[TheUmer]

Take this case:

A person sleeps for four or so hours and there's another person who lies down for four hours but he cannot sleep, yet he rests.

What's the real difference? The person who has slept feels fresh after getiing up which the other person feels heavy and fatigued. Why? Should we be unconscious to get rest?

 

[lzkelley]

Well, in that particular case i think both would feel pretty groggy...
In general, you do need to be unconscious - and not just that, but in a sleeping state. If you're not doing anything on your computer, that doesn't mean its turned off... it just means its not actively doing anything.

For your body to regenerate/recuperate, you need to be asleep - a lot of the important effects of sleeping are psychological/neurological, and don't work when you are awake.
Similarly shifts in your metabolism, etc won't happen until you're asleep.